Self-calibrating optical reflectance probe system

ABSTRACT

A self-calibrating optical reflectance probe system having an illuminant light source to illuminate a sample material, optical pickup means to collect reflected light from the sample material, and an articulated white reference reflection standard for illuminant reference to provide a system capable of accurately measuring optical reflectance and automated verification of proper operation. The probe system preferably employs an uncomplicated mount using a single pipe fitting and clamp.

This is a division patent application of co-pending U.S. patentapplication Ser. No. 10/711,129, filed Aug. 26, 2004, which claims thebenefit of U.S. Provisional Application No. 60/481,485, filed Oct. 8,2003.

BACKGROUND OF THE INVENTION

The invention relates to an optical reflectance probe system for theillumination of a sample material and detection of reflected light.

Optical reflectance measurements are commonly used for the analysis ofmaterials. In a typical optical reflectance system, light is shown uponthe material to be analyzed. An optical detector/measurement instrumentgathers some of the light reflected off of the material and measures theintensity of the light either at specific wavelengths or across aspectral range yielding a measurement of intensity versus wavelength.

Materials can be analyzed in this way for the presence of certainconstituents, the amount of these constituents, and the uniformity ofthese constituents throughout the consignment of the material. Specificuses include measurement of blend uniformity in pharmaceutical products,water or other solvent content in pharmaceutical products, measurementof protein, carbohydrates and water in agricultural products, and thepresence of foreign material in an otherwise homogeneous material suchas flour. Other applications include paint matching, quality control forpaper, textiles, packaging, food, pharmaceuticals and cosmetics.

Typically, an arrangement of a light source, lenses and mirrors are usedto align and project the illumination from the light source through aviewport window onto the sample material. Then additional lenses andmirrors are used to capture the light reflected from the sample materialand guide it to the optical detector/measurement instrument. Opticalfibers are also commonly used to guide the illumination light to thesample and/or optical pickup fibers to capture and guide the reflectedlight from the sample material back to the optical detector/measurementinstrument. Common light sources include incandescent and particularlytungsten-halogen lamps. Common optical detector/measurement instrumentsinclude photometers, monochronometers and optical spectrographs.

Optical reflectance measurement systems require calibration. Calibrationincludes the use of reflectance standards including white references,references with known spectral signatures, spectral line sources,transmissive filters, and shutters. Calibration generally takes placeduring manufacture of the optical detector/measurement instrument, andcommonly again after the system components are integrated. Calibrationof the system can change due to vibration, temperature change or otherconditions, so it is common to recalibrate periodically to ensure thesystem is performing within a required accuracy. In certainapplications, such as the production of pharmaceutical products, thereare government regulations requiring periodic verification ofperformance, and again, requiring the use of these calibrationstandards.

Current optical reflectance measurement systems require that some or allof these standards be employed by an operator dismounting the probe andmanually introducing these references for the system to sample. This canbe a cumbersome and time consuming task, as the system may be mounted ata point generally inaccessible. The unit could easily be damaged duringthe removal, or during reinstallation, requiring the system to berepaired, recalibrated, or worse, go unnoticed where data generated bythe system is relied upon to produce safe and effective product.

In process control or quality control applications, optical reflectancemeasurement systems are required to be adjacent to the sample materialbeing measured. Where the sample material is contained in a chamber,such as a vacuum chamber, mixer, blender or environmental chamber, theoptical reflectance probe must view the sample material through aviewport window. This window must withstand pressures, abrasion,chemical attack, and provide a seal between the probe and the chamberinterior, while providing a clear optical path for the probe to view.Further, mounts for the optical reflectance measurement system must beprovided to hold the probe in reference to the window to view the samplematerial within the chamber.

Current window and mount systems employ a flat viewport window and aseries of mounting brackets. The viewport window reflects some of theilluminant light from the probe back into the probes collecting optics,thus distorting the reflectance measurement. Anti-reflection coatings onthe window reduce but do not eliminate this back reflection. Further,these coatings cannot be applied to the inner surface of the windowbecause some of the coating may abrade off, contaminating the material,and generally cannot withstand chemical attack and other environmentalconditions. Other means to reduce effects caused by this back reflectionrequire complicated optical schemes including collimation and focusingoptics. The mounting brackets are generally custom for the particularchamber and optical reflectance probe being employed, and must bedesigned special for each application. Further, due to constraintsplaced by chamber geometry and the requirements of bracket position andorientation to the window, placement of the window at a desired viewingposition may not be possible for certain applications.

BRIEF SUMMARY OF THE INVENTION

The present invention provides a self-calibrating optical reflectanceprobe system having an illuminant light source to illuminate a samplematerial, optical pickup means to collect reflected light from thesample material, and an articulated white reference reflection standardfor illuminant reference. The probe system preferably has multipleilluminant light sources for redundancy and multiple optical pickupfibers for diversity in reflected light detection for more accuratemeasurements. Additional optional but preferred elements for the probesystem include an optical line source for wavelength calibration andverification, a spectral reference standard for dynamic rangeverification and/or wavelength calibration and verification, atransmissive filter for dynamic range measurement and a shutter for darkreference, a curved window to reduce reflected light from the windowsurface, and an uncomplicated mount preferably employing a singlesanitary pipe fitting and clamp (both preferably common to industry),which serves as the viewport as well and the probe mount, eliminatingthe need for additional brackets to mount the optical reflectance probeassembly. An additional fixture employing an integral curved window canbe welded onto a chamber containing the material to be detected, thusproviding a seal between the chamber and the probe assembly andsimultaneously providing the required mount for the probe assembly.These components can be used individually or severally to calibrate theoptical reflectance probe system and verify proper and accurateoperation without the removal of the system from its installation, andall by automation without the intervention of an operator. Thecomponents, including the reference standards, are preferably enclosedwithin the assembly so as to be sealed from contamination and protectedfrom damage due to handling.

It is therefore an object of the invention to provide an opticalreflectance probe system incorporating means that enables the system toself-calibrate and verify calibration without operator intervention.

It is another object of the invention to provide an optical reflectanceprobe system with a viewport window that reduces back reflection.

Yet another object of the invention is to provide an optical reflectanceprobe system with an uncomplicated mount using components common toindustry, eliminating the need for custom mounting brackets.

Still another object of the invention is to provide an opticalreflectance probe system having a viewport incorporated within a probemount to eliminate the need for additional mounting brackets.

The above and other objects, features and advantages of the inventionwill be apparent from the following description taken in connection withthe accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other objects and purposes of the invention will be bestunderstood in view of the following detailed description of theinvention taken in conjunction with the appended drawings, wherein:

FIG. 1 schematically shows a cross-sectional view of a self-calibratingoptical reflectance probe system and mount with a reference standardpositioned in a referencing position;

FIG. 2 is similar to FIG. 1 but shows the probe system and mount withthe reference standard positioned out of an optical path of the probesystem;

FIG. 3 schematically shows a cross-sectional view of the probe systemand mount of FIG. 1 with the probe system and mount separated to depictindividual components used for mounting; and

FIG. 4 shows an isolated end view of a transmissive filter and shuttercomponents and a spectral line source of the probe system of FIG. 1 inrelation to optical pickup fibers and an illuminant light source of theprobe system.

DESCRIPTION OF THE INVENTION

Referring to FIG. 1, a self-calibrating optical reflectance probe systemin accordance with a preferred embodiment of the invention is shown asincluding a probe housing 1 that encases components of the probe system.Seals 25 and 26 prevent contaminants from entering the probe system. Theprobe housing 1 has a threaded exterior 15, allowing a pipe fittingflange 14 to be adjusted along the length of the probe system toposition the end of the probe system at a desired distance from aviewport window 11. A locking ring is shown as preventing the pipefitting flange 14 from moving in relation to the probe housing 1. Agasket 16 and clamp 17 hold the probe system on a sanitary pipe fittingmount 12. The sanitary pipe fitting 12 is mounted by a weld 27 in a holecut in a chamber 13 where a material (not shown) is to be sampled. Thesanitary pipe fitting mount 12 houses the viewport window 11, which issealed against egress of the sample material by a seal 24. The seal 24can be made of an inert material such as Teflon® so as to notcontaminate any material in the chamber 13. The viewport window 11 ispreferably made of sapphire for abrasion resistance as well as chemicalresistance, again so as not to contaminate the sample material in thechamber 13. Within the probe system there are two sample illuminationlamps 3 and four optical pickup fibers 4 (two of which can be seen inFIG. 1) uniformly dispersed for diversity in sensing the reflected lightfrom the sample material. A white reference standard 7 is provided inthe form of a disk of diffuse reflective material, such as Spectralon®.This white reference standard 7 is mounted on an articulating mountrotatable on a bearing 28 and driven by a linkage 6 and actuator 5. InFIG. 1, the white reference standard 7 is shown in the “white reference”position, i.e., in an optical path through the probe system. Further, ashutter/filter wheel 18 is shown attached to an optic mounting plate 22.An electronic control module 9 controls all of activities of the lamps 3and actuator 5 via communications from an optical detector/measurementinstrument (not shown) of any suitable type. The back of the probesystem can be mounted to either a breakout box for communication andpowering the probe system as well as interconnecting to the opticalpickup fibers 4, or directly to the optical detector/measurementinstrument. FIG. 1 shows a mounting end 2 of a breakout box ordetector/measurement instrument attached with screws to the probehousing 1, such that the sanitary pipe fitting mount 12 is the singularmount for the probe system, or optionally a combination of the probesystem and optical detector/measurement instrument.

In FIG. 2, the probe system is shown in a material sampling mode withthe white reference standard 7 rotated into a position out of theoptical path of the probe system, such that light generated by thesample illumination lamps 3 is reflected back to the optical pickupfilters 4.

FIG. 3 shows the probe housing 1, gasket 16, clamp 17, and sanitary pipefitting mount 12 separated to more readily show how the probe system ismounted.

FIG. 4 depicts an end on view of the optic mounting plate 22, showing apreferred arrangement for the sample illumination lamps 3, opticalpickup fibers 4, and shutter/filter wheel 18, the latter of which ismounted for rotation on bearings 19 and driven by an actuator (notshown). FIG. 4 further depicts individual shutters 20, open apertures30, and individual transmissive filters 21. Also, the mounting positionfor a spectral line source 29 is shown.

Operation of the probe system will be described in reference to theFigures. During operation, only one of the illumination lamps 3 need bepowered (the other being provided for redundancy) to illuminate thewhite reference standard 7 (FIG. 1), whose diffuse reflectance of theilluminant is partially captured by the optical pickup fibers 4. Thelight captured by the optical pickup fibers 4 is processed and used as ahigh level (white) reference signal. The white reference standard 7remains in this position, preventing light passing through the window 11from reaching the pickup fibers 4. The illumination lamp 3 is thenturned off or the pickup fibers 4 are shuttered by rotating theshutter/filter wheel 18 to position the shutters 20 over the pickupfibers 4. A dark signal captured by the pickup fibers 4 at this time isprocessed and used as a low level (dark) reference signal.

Further testing of the system can be administered by rotating theshutter/filter wheel 18, positioning the transmissive filters 21 overthe optical pickup fibers 4, again with the white reference standard 7deployed and the illumination lamp 3 powered. Depending on the filterchosen for the transmissive filters 21, stray light can be measurementor spectral accuracy verified. If a time-integrating opticaldetector/measurement instrument (such as a photo detector array basedspectrograph) is employed, system linearity can be measured by deployingthe white reference standard 7 and the illumination lamp 3 powered andthe shutter/filter wheel 18 positioning the open apertures 30 over theoptical pickup fibers 4, then sampling the captured light at varyingintegration times set in the optical detector/measurement instrument.Spectral resolution and accuracy can be measured by deploying the whitereference standard 7 while the illumination lamp 3 is de-powered, theshutter/filter wheel 18 positions the open apertures 30 over the opticalpickup fibers 4, and the spectral line source 29 is powered. Light fromthe spectral line source 29 will reflect off the white referencestandard 7 and a potion thereof is subsequently captured by the opticalpickup fibers 4. The light captured by the optical pickup fibers 4 canbe processed yielding both spectral accuracy and spectral resolution.

During material sampling, the white reference standard 7 is retracted asshown in FIG. 2, the illumination lamp 3 powered, and the shutter/filterwheel 18 positioned such that the open apertures 30 are over the opticalpickup fibers 4. Light from the illumination lamp 3 passes through adust window 10 and again through the viewport window 11 onto the samplematerial within the chamber 13. The dust window 10 and viewport window11 have curvatures such that their inner and outer curvatures arespherical and their inner and outer center of curvatures aresubstantially at the same locus point. Further, the center of curvaturesof the dust and viewport windows 10 and 11 are positioned substantiallyat the level of the lamps 3 and on center with the probe system. Thisarrangement maintains minimal effect on the light passing through thewindows 10 and 11, while all light reflected from the lamp 3 by thesurfaces of the windows 10 and 11 is to a great degree projected back tothe lamps 3 and away from the optical pickup fibers 4. This arrangementalso provides greater structural strength for the viewport window 11,allowing for higher loads or a thinner window 11 for an existing loadspecification. Additionally, the curved shape allows sample material tomore easily fall away from the window 11, and enables sample material tobe blown clean from the window 11 with an air jet to a greater degreethan a flat window would allow. Light passing through both windows 10and 11 and reaching the sample material is reflected back through thewindows 10 and 11, where some of the reflected light is captured by theoptical pickup fibers 4. This light is then processed by the opticaldetector/measurement instrument and, with information gained from thewhite reference and dark reference signals, yields information about thesample material itself.

If in operation, the lamp 3 being used fails, the second lamp 3 can bepowered and a new white reference signal generated using the processoutlined above to again ready the system for material sampling. Thisswitching of lamps 3 and all testing described above can be automatedand performed without operator intervention. Documentation on testresults required by regulatory agencies can also be automaticallygenerated, again without operator intervention. In systems employingmore than one probe system, each probe system can have the capability ofdetermining itself unhealthy and report this to the system gatheringdata, which would then take appropriate action, such as calling servicefor the probe system that declared itself unhealthy and not using datagathered from the unhealthy probe system.

In a variation of this system, a second reference standard could beinstalled with a second actuator to employ a reference standard with aknown spectral signature. In operation of this embodiment, the whitereference standard 7 would be retracted, the second reference standarddeployed, the illumination lamp 3 powered and the shutter/filter wheel18 positioned such that the open apertures 30 are over the opticalpickup fibers 4. Light captured by the optical pickup fibers 4 is thenanalyzed for spectral signature, both wavelength accuracy and absorptionlevel accuracy.

While the invention has been described in terms of specific embodiments,it is apparent that other forms could be adopted by one skilled in theart. For example, the probe system and its components could differ inappearance and construction from the embodiments shown in the Figures,and appropriate materials could be substituted for those noted.Accordingly, it should be understood that the invention is not limitedto the specific embodiments illustrated in the Figures. It should alsobe understood that the phraseology and terminology employed above arefor the purpose of disclosing the illustrated embodiments, and do notnecessarily serve as limitations to the scope of the invention.Therefore, the scope of the invention is to be limited only by thefollowing claims.

1. A mount for an optical reflectance probe system, the mount consistingessentially of a single sanitary pipe fitting and clamp.
 2. The mountaccording to claim 1, wherein the mount further comprises an integralviewport window.
 3. The mount according to claim 2, wherein the viewportwindow comprises a curved surface to reduce reflected light from thewindow.
 4. A mount for an optical reflectance probe system, the mountcomprising: a housing containing the optical reflectance probe systemand having an integral curved viewport window to reduce reflected lightfrom the window surface; and an assembly on the housing for mounting thehousing, the assembly comprising a single sanitary pipe fitting andclamp.